With the increase in integration degree of semiconductor devices, patterns required in semiconductor devices shrink in size with each generation of integration. A fundamental trend in the field of integrated circuits is the ever-decreasing reduction of critical feature sizes. Smaller feature sizes enable the increase in density of active devices for a given chip area, resulting in greater functionality and lower manufacturing cost. Smaller feature sizes also improve device performance. A critical dimension (CD) for a photolithography process defines a minimum line width, or a contact hole, or the smallest space between two lines or two holes of a masking material that may be patterned by the process. Double patterning processes can provide smaller pitches than single patterning processes.
FIG. 1A shows a layout diagram of a reticle in a double patterning process. A target pattern can be obtained in a material by etching (patterning) the material twice using a reticle 101 and a reticle 102. However, a problem with the double patterning process is that the process window of the pattern formed by the patterning is small, i.e., fault tolerance of the pattern is relatively small, which causes a relative large deviation between the patterned pattern and the expected pattern. For example, the portion of the pattern indicated by a circle may be broken (open), causing the patterned pattern to deviate from the target pattern.
There are two conventional techniques to increase the patterning process window. FIG. 1B shows a technique that increases the area size of a portion of the reticle 101 that may deviate from the target pattern. However, this technique would cause the reticle 101 to extend into the space of the reticle 102, and the resulting patterned pattern would still not conform to the target pattern.
FIG. 1C shows another technique that utilizes a sub-resolution assist feature (SRAF) pattern 103 that is disposed next to the main pattern of the reticle 101, which may deviate from the target pattern. In this technique, the SRAF pattern 103 is provided on the reticle 101, and its size is strictly limited. After the lithography process, the SRAF pattern 103 is not transferred to the material to be patterned. This approach only provides a limited increase in the lithographic process window.